In the packaging of foods which may release blood, such as meat or fish, there is a problem in providing packages which can isolate and hide the blood released by the food, since it has been found that its presence and visibility inside the package renders the latter unattractive to the purchaser.
Amongst the solutions proposed for this problem in the prior art, the simplest consists of the provision of a tray provided with a pad or layer of absorbent material, generally paper. Solutions of this type are described, for example, in patent applications EP-A-O 182 139, GB-A-1 168 925, EP-A-0 544 562, in patent FR 2 688 474, and in utility model DE 9013898.8.
The absorbent layer may simply be fixed to the internal surface of a tray of plastics material or interposed between two sheets of plastics material, of which that which is in contact with the food has holes through which the liquid released by the food is conveyed towards the interposed absorbent layer.
The trays mentioned above have the disadvantage of being made of materials of different kinds, that is, expanded plastics, generally polystyrene, and paper or similar hydrophilic materials, which are difficult to separate from one another so that there is little or no chance of their being recycled. Moreover, owing to the presence of the paper, the production cost of these trays is considerably greater than that of conventional non-absorbent trays.
Trays made of a single material, generally expanded polystyrene, which can isolate the liquid exuded from the food, causing it to descend by gravity into a space formed between two sheets of the aforesaid plastics material by passing through holes formed in the sheet on which the food is placed are also known. Examples of these trays are given in patent applications EP-A-0 574 819 and WO 94/00366.
Trays of the type described above have the disadvantage of necessarily having quite large holes in order to allow the blood to descend by gravity and, for this reason, leave the blood which has collected in the space easily visible. Moreover, the blood can easily return to the sheet which is in contact with the food as a result of the inversion or simply the inclination of the tray.
The technical problem upon which the present invention is based is that of providing a tray which is made solely of plastics material and which itself has the capacity to absorb liquids, particularly aqueous liquids, released by foods, thus preventing the problems set out above with reference to the trays of the prior art.
A partial solution to the aforementioned technical problem was offered in patent application EP 0 090 507, which describes a "fast food" container produced from a composite structure constituted by two superimposed sheets of which one is a normal sheet of closed-cell expanded plastics material and the other is a sheet of substantially open-cell plastics material which has the ability to absorb condensed steam coming from the foods present in the container.
The sheet of substantially open-cell plastics material constitutes the inner layer of the container which is in direct contact with the food.
The aforesaid open-cell sheet has a high density (150-450 g/l), however, and its ability to absorb water is not adequate for the purposes of the present invention.
The open-cell sheet produced according to the method of the aforementioned patent application also has an irregular surface which impairs the appearance of the tray to a certain extent.
According to application EP 0 090 507, the open-cell sheet is produced, by the mixing of polystyrene with an excess of a chemical nucleating and expanding agent such as sodium bicarbonate/citric acid, in an extruder having a single mixing chamber and a single screw, the molten mixture being brought to a temperature of 196-204.degree. C. immediately before extrusion.